The present invention relates to a light beam control apparatus, typical examples of which are a laser printer and a laser-operated figure drawing machine, and also relates to a linear scale device suitable for detecting the scanning position of a laser beam used in the light beam control apparatus.
In a laser printer or laser-operated figure drawing machine, a scanning operation is carried out with a laser beam in accordance with recording signals, to form a desired picture (including characters etc.) on a predetermined medium. In order to form a picture high both in accuracy and in picture quality, it is essential to control the scanning position of the laser beam by detecting it accurately.
For the purpose of controlling the scanning position of a laser beam, heretofore the angle of rotation of a mirror deflecting the laser beam is detected, or with respect to a predetermined point on the scanning line the scanning position is detected in a time division mode.
That is, in such a device, the scanning position of the laser beam is indirectly detected and controlled, and therefore improvement of the accuracy and resolution is limited.
In order-to overcome this difficulty, an apparatus has been proposed in the art in which the laser beam is divided into a main scanning beam and a monitor scanning beam; and while the main scanning beam is deflected, the monitor scanning beam is also deflected with one and the same scanning means so that the two beams are substantially synchronized with each other; and instead of the scanning position of the main scanning beam, the scanning position of the monitor scanning beam is detected.
FIG. 1 shows the arrangement of a detecting device for detecting the scanning position of the monitor scanning beam in such an apparatus. In FIG. 1, reference numeral 1 designates a plurality of optical fibers. First ends 2 of the optical fibers 1 are arranged in parallel with the scanning line of the monitor scanning beam, and the remaining ends are connected to respective elements in a photodetector 3. The monitor scanning beam is picked up by the optical fibers 1 and introduced to the photodetector 3, so that the photodetector 3 produces an output corresponding to incident light beams thereto. Thus, the scanning position of the monitor scanning beam (and accordingly the scanning position of the main scanning beam synchronous with the monitor scanning beam) can be detected from the output of the photodetector 3. Therefore, the scanning position of the main scanning beam on the predetermined medium can be controlled by controlling a scanning means such as scan mirror according to the output of the photodetector 3.
As was described above, the conventional apparatus employs a plurality of optical fibers, and therefore it follows that the apparatus is large in the number of components, high in manufacturing cost, and unavoidably bulky.
FIG. 2 shows the arrangement of a conventional laser-operated figure drawing machine of another type. In the machine, a laser beam generated by a laser beam generator 11 is applied to a half-mirror 12, where it is divided into first and second beams, namely, a main beam and a monitor beam. The first beam (main beam) is applied to a modulator 13, where it is modulated with a recording signal. The main beam thus modulated is expanded by a beam expander 14, and then applied to a rotary polygonal mirror 15. The main beam reflected by the polygonal mirror 15 is applied to an f.theta. lens 16 where the scanning speed is corrected, and the main beam thus processed is applied through a mirror 17 to a recording surface 18. The recording surface 18 is moved in an auxiliary scanning direction (for instance in a horizontal direction in the paper surface of FIG. 2) while the main beam is deflected by the rotary polygonal mirror 15 in a main scanning direction (for instance in a direction perpendicular to the paper surface of the drawing). Therefore, an image is formed on the recording surface 18 in correspondence to the recording signal.
On the other hand, the second beam (or monitor beam) is applied to the beam expander 14 without passing through the modulator 13. Instead, the monitor beam is reflected by a plurality of mirrors (omitted in FIG. 2). Then, the monitor beam is applied through the rotary polygonal mirror 15, the f.theta. lens 16, the mirror 17, and a mirror 19 to a linear scale 20. The reason why only the monitor beam is received by the mirror 19 is that the monitor beam has been spatially separated from the main beam by the optical system disposed before the polygonal mirror 15. The linear scale 20, as shown in FIG. 3, has a number of slits 20A arranged along the monitor scanning direction. Therefore, as the monitor beam is deflected in the main scanning direction by the rotary polygonal mirror 15 in synchronization with the main beam, it is turned on and off by the slits 20A, thus being formed into a pulsed beam in the form of a train of pulses. The monitor beam thus processed is collected by a light guide 21 provided behind the linear scale 20, and is then applied to a photodetector 22. Accordingly, the photodetector 22 outputs pulses in correspondence to the scanning operation in the main scanning direction. Thus, for instance by counting the output pulses, the scanning position of the beam in the main scanning direction can be detected.
As was described above, in the conventional figure drawing machine, it is necessary to provide the light guide 21 behind the linear scale 20 to transmit the light beam to the photodetector 22. Therefore, it follows that the apparatus is large in the number of components, intricate in construction, and bulky.